Sen. Dean Heller’s surprise announcement on Friday that he opposes the Senate health-care bill as written underlines the complex pressures he faces as he heads into a tough re-election bid next year, and it also highlights the challenges confronting Republican Senate leaders.

Mr. Heller, standing alongside Nevada’s Republican Gov. Brian Sandoval at a press conference in Las Vegas, said he couldn’t lend his support to the GOP bill in its current form because of its far-reaching changes to the Medicaid program.